Radio-Frequency (RF) power harvesting techniques have been applied widely for providing long term power supply of low energy consumption and battery-less devices. It is an essential component of passive RFID systems. Recently, there has also been a strong interest in implementing wireless power harvesting techniques with sensor technology for monitoring various environmental conditions such as temperature, pressure, humidity and human health conditions.
Providing power to wireless sensors is typically addressed with onboard batteries. This is acceptable for small scale deployments where a handful of sensors can be regularly serviced by a technician. However, for large commercial applications in retail stores, for which there might be hundreds of sensors per location, this maintenance quickly becomes untenable.
Next generation sensor networks may be powered by energy harvesting techniques to avoid requiring battery maintenance. Energy harvesting is a process by which energy is derived from external sources (e.g., radio frequency energy, solar power, thermal energy, wind energy, salinity gradients, or kinetic energy), captured and stored.
Energy may be harvested from radio frequency signals propagating wirelessly. With RF harvesting, wireless energy comes from a radio frequency transmitting device that is some distance away from a device that harvests energy from the radio frequency transmission. Properties of an energy harvester include its ability to harvest energy efficiently from available RF signals, its ability to store the harvested energy efficiently with minimal storage loss, and its ability to make the stored energy available to meet the voltage, current, and duty cycle requirements of a desired application.
One of the more popular forms of RF used today is Wi-Fi (also referred to as IEEE 802.11a/b/g/n etc.) communications. Today, most Wi-Fi communications are in the 2.4 GHz and 5.8 GHz frequency bands and there are many local area networks that are based on Wi-Fi in which access points enable Wi-Fi clients to gain access to networks such as the Internet. Furthermore, the 2.4 GHz and 5.8 GHz bands also support other networking standards, such as Zigbee and Bluetooth, and other proprietary networks, each transmitting energy by communicating in this same frequency band.
Recent work has shown that Wi-Fi energy is abundant in a typical office environment, although at low power levels, e.g. yielding below −20 dBm at the feedpoint of a half-wavelength, 6 dBi gain patch antenna. Harvesting energy from ambient Wi-Fi has been the subject of several recent investigations.